In recent years, research into the area of emissions from coal-fired utility boilers, cement plants, mineral ore processing, steel-making, gypsum wallboard production and waste-to-energy plants has shown that a large fraction of the input mercury (present as trace species in the feed materials) is emitted in the exhaust gases. Unlike most other metals, a large portion of the mercury volatilized into the vapor phase during high temperature processing does not condense onto particulates at the lower temperatures typically present in pollution control devices such as electrostatic precipitators and fabric filters. Therefore, it cannot be collected and disposed of along with particulate ash like the other metals. To complicate matters, mercury can exist in its oxidized (Hg2+) form, principally as mercuric chloride, (HgCl2), or in its elemental (Hg0) form as vaporous metallic mercury. The relative amount of each species appears to depend on several factors such as fuel type, plant operating conditions, the type of particulate collector installed, and various other factors.
In addition to particulate removal, several industrial processes also include scrubbers for removal of acid gases such as HCl and sulfur dioxide. HCL is typically removed by dissolution in water. The resulting liquor is then neutralized with a substance such as lime, and the solids are disposed of. One of the methods to remove sulfur dioxide is to employ a wet flue gas desulfurization (FGD) scrubber, wherein the flue gas containing sulfur dioxide is contacted with a spray of water droplets containing an alkaline component such as limestone, lime, dolomite, magnesium compounds or sodium compounds. In the scrubber, the alkaline component, such as limestone (CaCO3), reacts with the sulfur dioxide (SO2) to form a neutral compound such as gypsum (CaSO4.2H2O). Oxidized mercury (mainly mercuric chloride) present in the flue gas, which is highly water soluble, was expected to be captured with very high efficiency, while elemental mercury in the flue gas, which has a very low solubility in water, was expected to be captured with a very low efficiency. However, it has been determined that the elemental mercury has a negative capture efficiency, suggesting that a portion of the incoming oxidized mercury is reduced in the FGD system, and re-emitted as elemental mercury in the exhaust gases.
Field measurements carried out at several full-scale units demonstrate that elemental mercury concentrations do indeed increase across the wet FGD system for all cases (see Table 1), indicating re-emission of mercury across the FGD vessel.
TABLE 1Elemental Mercury Concentrations across Wet FGD Scrubbers(Miller et al. August 2006)FGDHg Removal if noFGD InletOutlet Hg0Hg0Total HgRe-emission HadYearHg0 Conc.Conc.Increase,RemovalOccurredSiteSampledμg/Nm3μg/Nm3μg/Nm3%%With upstream Selective Catalytic Reduction (SCR) SystemS220010.40.90.58995S220020.31.31.08496S420010.50.80.39094S420021.01.30.39193S520020.71.00.39193Without SCRS220013.45.01.65167S420015.67.11.54657S420025.78.02.34460S520024.76.11.45162
As the proportion of coal-fired power plants that will be equipped with wet FGD systems increases, correspondingly, the amount of FGD by-product solids (mainly gypsum) from these scrubbers also increases; it has been projected that FGD gypsum production will increase from 31 million tons in 2004 to 86 million tons by 2020. Continued regulatory characterization of FGD by-products as non-hazardous is a critical factor for its marketability for beneficial end-use, such as gypsum wallboard production, as well as for disposal. This means that the amount of mercury in FGD solids must be minimized, and any remaining mercury in the FGD solids must be rendered stable.
Nolan et al. (U.S. Pat. No. 7,037,474) describe the addition of aqueous sulfide species to the wet FGD scrubber solution to precipitate the soluble mercury species as a mercury sulfide precipitate. One of the disadvantages of this approach is that the aqueous sulfide species reacts with the scrubber liquids to form hydrogen sulfide (H2S) gas, creating objectionable H2S emissions, and objectionable odor. Another disadvantage is that a portion of the mercury sulfide that is precipitated reports to the gypsum phase, thus making the mercury available for leaching or thermal desorption when the gypsum is subsequently disposed or used.
Researchers have also tested an organo-sulfide compound, such as Degussa Corporation's TMT-15 (tri-mercapto-s-triazine, tri-sodium salt), to prevent re-emission of elemental mercury. TMT-15 has been used to prevent mercury re-emissions from wet FGD systems on municipal waste incinerators. Full-scale testing of TMT-15 at two coal-fired power plants showed only limited effectiveness, and in some cases, increased mercury re-emissions (Currie et al., Air Quality VI Conference, September 2007, Arlington, Va.).
US Patent Application publication 2005/0155934 describes a method and material for removing contaminants, such as mercury, from a flue gas stream using a magnetic removal adsorbent material that may be recovered and reused. The preferred adsorbent material comprises an iron-bearing activated carbon. The method introduces the mercury sorbent into the flue gas, and so does not address the issue of re-emission of mercury from a scrubber, nor separation of the mercury adsorbent from other solid components in the slurry.
US Patent application publication 2006/0076229 describes the use of magnetic powdered activated carbon to remove contaminants such as mercury from fluid streams including flue gases from a combustion plant. The method introduces the mercury sorbent into the flue gas, and so does not address the issue of re-emission of mercury from a scrubber, nor separation of the mercury adsorbent from other solid components in the slurry.